1. Field of the Invention
The present invention relates to a communication system which transmits and receives data using a digital subscriber line (DSL), and more particularly, to a communication system capable of controlling amplitude of a received signal, and a controlling method thereof.
2. Discussion of Related Art
In recent years, as demands increase for broadband communications such as on the Internet and personal computers are widely used, there is a need for a communication method that high speed data communications can be realized and installation costs and usage costs can be reduced. To meet these demands for high speed communications, there has been provided an xDSL (digital subscriber line and its variations) communication method that digital data communications are performed using a general copper telephone line installed in every office and at home.
An xDSL communication system, which generally indicates all types of communication method using the telephone line, includes a High data-rate DSL (HDSL) system substituting for T1 line, an Symmetric DSL (SDSL) system substituting for T1 or E1 using a single twist-pair copper line, an Asymmetric DSL (ADSL) system capable of transmitting a large amount of data in a public switched telephone network (PSTN) environment, and so on.
FIG. 1 is a block diagram illustrating an ADSL modem used in an ADSL system. An ADSL modem 10 includes an amplifier 11 for amplifying a signal received through a telephone line 1, an analog-to-digital converter (ADC) 12 for converting an analog signal outputted from the amplifier into a digital signal, a digital signal processor (DSP) or discrete multi-tone (DMT) controller 13, and a host controller 14. The DMT controller 13 demodulates a modulated digital signal in a DMT way to restore an original signal, which is provided to the host controller 14, and modulates a signal outputted from the host controller 14 in the DMT way. The host controller 14 controls a general operation of the ADSL modem 10. The ADSL modem 10 further includes a digital-to-analog converter (DAC) 15 for converting a digital signal outputted from the digital signal processor 13 into an analog signal, and an amplifier 16 for amplifying the analog signal outputted from the digital-to-analog converter 15 up to a predetermined level and outputting an amplified analog signal to the telephone line 1.
A maximum distance between a central office and a remote terminal in which a signal can be normally transmitted without using a repeater in an ADSL system is 5.5 Km. Owing to noise on a telephone line and attenuation according to the distance, although the central office transmits a signal with a constant intensity, the intensity of the signal which arrives at the ADSL modem 10 varies with the distance between the central office and the ADSL modem 10. In other words, an ADSL modem located from a distance to the central office receives a weaker signal than an ADSL modem located near the central office. Accordingly, the ADSL modem 10 is essentially provided with the amplifier 11 for amplifying the signal received through the telephone line 1.
However, a gain of the amplifier 11 provided for the ADSL modem is generally fixed at a constant value with no relation to the distance between the central office and the ADSL modem 10. Accordingly, if the gain of the amplifier 11 is set to a large value, the ADSL modem located a large distance from the central office can compensate for the attenuation due to the distance, thereby restoring an original signal. Whereas the amplitude of a signal outputted from the amplifier of the ADSL modem located near the central office may not be within a quantization range (+L to −L) of the analog-to-digital converter 12. As a result, it is impossible for the ADSL modem located near the central office to correctly restore the original signal.
FIG. 2 illustrates exemplary waveforms of signals outputted from the amplifier 11 of the ADSL modem 10. In the case where the central office transmits a signal with a constant intensity, and ADSL modems are disposed at different distances from the central office, intensities (e.g., amplitude) of signals received at the ADSL modems are different depending on the distance between the central office and the respective ADSL modem 10. For instance, a signal that is outputted from the amplifier 11 of the ADSL modem located near the central office is referred to as (c), and a signal that is outputted from the amplifier 11 of the ADSL modem located from a distance to the central office is referred to as (a). When the quantization range is between +L and −L, signals, for instance, signals (a) and (b), of which maximum amplitude stay within the quantization range (+L to −L) can be correctly restored to the original signal by the digital signal processor 13. However, a signal, for instance, signal (c) deviating from the quantization range (+L to −L) cannot be correctly restored to the original signal. This problem is not limited to the ADSL modem but is applicable to all xDSL type modems.
Signal modulation technologies of the xDSL modem generally include a Carrierless Amplitude Phase (CAP) Modulation, a Discrete Multi-tone (DMT) method, a 2Binary 1Quaternary (2B1Q) method, and a Quadrature Amplitude Modulation (QAM). The QAM method is also called a vertical amplitude modulation, and was first developed in the middle of 1990s by the AT&T Bell research center, and adopted as an analog signal transmission standard at the American National Standards Institute (ANSI), the European Telecommunications Standards Institute (ETSI), the International Telecommunications Union (ITU). Since the QAM method uses a band below 3,300 Hz, it is characterized in that it can be used for exchange line or released line and management of multipoint network is available.
The CAP method is a kind of time division methods. One of the first CAPs was commercialized by the BELL Atlantic company of the U.S. in 1993, and it separates and uses a single carrier wave signal sound. Typically, the CAP modulates a digital vertical signal sound and uses two digital cross line band filters having a phase difference of P/2 by the same amplitude and phase response. The CAP method has advantages such as wide bandwidth and ease of application.
The 2B1Q method is a digital transmission method that is widely being used in HDSL, CSU, ISDB transmission devices, an SDSL system, and so forth. The 2B1Q has a characteristic of symmetric transmitting and receiving speed. The 2B1Q method which typically modulates the amplitude of a single wave to transmit data was adopted as transmission standards in the ANSI, ETSI and ITU.
The DMT method, which was first commercialized by the Bell Core company of U.S. in 1993, was adopted as standard of digital transmission in the ANSI, ETSI and ITU. The DMT method is a parallel transmission technology using a plurality of narrow band carrier waves, and it has a better noise suppression function and a less interference phenomenon on other communications line than the CAP method. According to ANSI T1.413 regulation of the DMT, a frequency band of 0-1.104 MHz is divided into 256 sub-channels each having the same size, in which 26 kHz (#6) to 134 kHz (#31) are allocated to an upstream and 142 kHz (#33) to 1.100 kHz (#255) to a downstream. Attenuation of the copper line is low in a relatively low frequency band, and a signal to noise ratio (SNR) is good. Generally, the DMT is used in a communication system at a configuration higher than 10 bits/Hz. In case that the line is not at good configuration (e.g., approximately less than 4 bits/Hz), the DMT accepts signals of relatively low SNR to lower the modulation function and remove unnecessary noise.
As described above, the DMT modulation method divides a frequency channel in use into sub-channels each having the same sized interval (e.g., 4 kHz). Here, each of divided sub-channels is called “tone.” When communications start between a central office and an ADSL modem, the central office transmits signals having the same intensity to tones, respectively. However, since a signal passing through communication lines (or telephone line) is influenced by an Additive White Gaussian Noise (AWGN), intensities of the signals received in the ADSL modem are different at every tone. To this end, it is not clear whether the received signal at the ADSL modem is a real ADSL signal or noise.
As such, it is desirable to develop a method for determining whether a signal received at the ADSL modem is noise or a real xDSL signal by controlling a gain of an amplifier depending on amplitude of a signal received at an ADSL modem. Therefore, there exists a need for an xDSL modem capable of controlling a gain of an amplitude depending on amplitude of a received signal and of determining whether a signal received at an ADSL modem is noise or a real xDSL signal.